The investigation of novel imaging systems for nuclear medicine and molecular imaging continues to be the long-term objective of this project. For the previous several years investigations have focused on Compton scatter cameras and their potential for providing significant improvements over conventional absorbing collimation for single photon imaging. Results so far indicate that Compton cameras will provide superior performance at energies >240keV or when operated in a Compton probe arrangement. Nevertheless, work is still required to create large arrays of scattering detectors having energy resolution approximately 1keV FWHM. In the this project period, investigations will change directions slightly and focus on developing the very high resolution (VHP) PET concept, which was conceived as result of the Compton camera investigations and reported in the last renewal. Recently, 0.8mm FWHM spatial resolution has been demonstrated using this technique and the goal of this investigation is to evaluate performance of a practical device relative to present small animal PET technologies and to develop technologies necessary to allow molecular imaging of small animals with resolution limited only by the positron range at efficiencies approaching 10%. The major Specific Aims of the investigation are: 1. Evaluate the potential for the VHR PET concept to outperform PET instruments of more conventional design. This will be accomplished by using statistical performance evaluation methods based on the Cramer-Rao bound. 2. Construct a PET test bench upon which the new VHR PET technologies will be tested. 3. Further develop the high-resolution solid-state detector and readout technologies necessary to establish feasibility of the concept. Work will be done in a collaboration that includes the University of Michigan, Ohio State University, CERN (Geneva), the University of Valencia (Spain), and the University of Ljubljana (Slovenia). Results of this investigation will not only be a compelling demonstration of the VHR PET concept but will also provide technologies necessary to advance both VHR PET and Compton cameras. [unreadable] [unreadable] [unreadable]